Worm wheel, worm reduction gear, and method for producing worm wheel

ABSTRACT

An inner wheel element (15a) is embedded in an outer wheel element (16a), such that a continuous range from an inner diameter side circumferential surface configuring an inner surface of a first annular concave part (22), through an outer circumferential surface of the inner wheel element (15a), to an inner diameter side circumferential surface configuring an inner surface of a second annular concave part (38) in a surface of the inner wheel element (15a) is covered over the entire circumference. Accordingly, a structure is achieved which easily secures a holding power of the synthetic resin outer wheel element with respect to the inner wheel element.

TECHNICAL FIELD

The present invention relates to a worm wheel which includes an innerwheel element which serves as a core material and a synthetic resinouter wheel element having a tooth part, a worm reduction gear includingthe worm wheel, and a manufacturing method of the worm wheel.

BACKGROUND ART

FIGS. 20 to 25 illustrate one example of an electric power steeringdevice which is described in Patent Document 1 and the like and is knownfrom the past. A front end part of a steering shaft 2 in which asteering wheel 1 is attached to a rear end part is rotatably supportedin a housing 3. A worm wheel 4 is fixed to a portion driven rotationallyby the steering shaft 2. On the other hand, a worm shaft 6 is connectedto an output shaft of an electric motor 5. Further, a worm tooth part 18provided in an outer circumferential surface of an axial intermediateportion of the worm shaft 6 is engaged with a worm wheel tooth part 19provided in the outer circumferential surface of the worm wheel 4, sothat a predetermined magnitude of the auxiliary torque (auxiliary power)can be applied with respect to the worm wheel 4 from the electric motor5 in a predetermined direction.

The worm wheel 4 is externally fitted and fixed to an axial intermediateportion of the output shaft 7 which serves as an output part of theauxiliary torque, and is rotated together with the output shaft 7. Theoutput shaft 7 is coupled with the front end part of the steering shaft2 through a torsion bar 9 in the housing 3 in a state where a portionnear both ends of the axial intermediate portion is supported to berotatable only by one pair of rolling bearings 8 a and 8 b. The electricmotor 5 rotationally drives the worm shaft 6 according to a directionand a magnitude of a steering torque which is detected by a torquesensor 10 and is applied from the steering wheel 1 to the steering shaft2, and the auxiliary torque is applied to the output shaft 7. Therotation of the output shaft 7 is transmitted to a pinion shaft 14 whichserves as an input part of a steering gear unit 13 through one pair ofuniversal joints 11 a and 11 b and an intermediate shaft 12, and adesired steering angle is given to a steering wheel.

In the case of the illustrated example, the worm wheel 4 is formed bycombining a metal inner wheel element 15 which serves as a core materialand a synthetic resin outer wheel element 16. That is, in the worm wheel4, the portion which is externally fitted and fixed to the output shaft7 serves as the metal inner wheel element 15 having a circular ringshape, and the portion including the worm wheel tooth part 19 serves asthe synthetic resin outer wheel element 16. Further, as described above,the outer wheel element 16 is made of a synthetic resin, so as tofacilitate an operation (cost reduction) that forms the worm wheel toothpart 19 in the outer circumferential surface of the worm wheel 4, and toreduce a tooth hitting noise generated in the engaging portion betweenthe worm tooth part 18 of the worm shaft 6 and the worm wheel tooth part19 of the worm wheel 4.

The outer wheel element 16 is made of a synthetic resin, and a radiallyouter end part of the inner wheel element 15 is embedded therein overthe entire circumference through an injection molding (insertionmolding). In the outer circumferential surface of the inner wheelelement 15, a (gear-shaped) concave-convex part 17 in a circumferentialdirection is provided, and a portion of a synthetic resin configuringthe outer wheel element 16 enters into a plurality of concave partsconfiguring the concave-convex part 17, so as to improve a holding powerof the outer wheel element 16 in a rotation direction with respect tothe inner wheel element 15.

In the case of the above-described structure in the related art, onlythe radially outer end part of the inner wheel element 15 is embedded inthe outer wheel element 16, so as to secure the holding power of theouter wheel element 16 with respect to the inner wheel element 15. Forthis reason, there is still room for improvement in the holding power.

Incidentally, in the case of the above-described structure in therelated art, the concave-convex part 17 in the circumferential directionis provided in the outer circumferential surface of the inner wheelelement 15, and the portion of a synthetic resin configuring the outerwheel element 16 enter into the plurality of concave parts configuringthe concave-convex part 17. For this reason, in the outer wheel element16, the portion which is superimposed on the radially outer side withrespect to the concave-convex part 17 may have different the radialthickness for each of portions in which a plurality of teeth 20 and 20configuring the worm wheel tooth part 19 are positioned (see FIGS. 24and 25). In this case, the molding shrinkage amount during the injectionmolding is different (is large in a portion (for example, α portion ofFIG. 25) having a large radial thickness, and is small in a portion (forexample, β portion of FIG. 25) having a small radial thickness) for eachof portions where the plurality of teeth 20 and 20 are positioned. Thus,a difference occurs in sizes of the plurality of teeth 20 and 20 aftermolding, so that a manufacturing error such as a pitch error may occurin the worm wheel tooth part 19.

RELATED ART REFERENCE Patent Document

Patent Document 1: JP-T-2013-084613

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The invention is made in consideration of the above-described situation,and an object thereof is to implement a structure which can improve aholding power of a synthetic resin outer wheel element with respect toan inner wheel element which serves as a core material.

Means for Solving the Problems

A worm wheel of the invention includes an inner wheel element and anouter wheel element.

The inner wheel element has a first annular concave part provided to berecessed in an axial direction in an axial one-side surface.

The outer wheel element is made of a synthetic resin, and has a wormwheel tooth part in an outer circumferential surface, and the innerwheel element is embedded in the outer wheel element, such that acontinuous range from an inner diameter side circumferential surfaceconfiguring an inner surface of the first annular concave part throughan outer circumferential surface of the inner wheel element to a portion(positioned radially inside compared to the radial intermediateposition) near a radially inner end of an axial other-side surface ofthe inner wheel element in a surface of the inner wheel element iscovered over an entire circumference.

Incidentally, in a case where the invention is implemented, metal may beadopted as a material of the inner wheel element. However, for example,a synthetic resin may be adopted which is excellent in the thermalresistance compared to the material of the outer wheel element andhardly receives heat effect when the outer wheel element isinjection-molded (for example, thermoplastic resin is adopted as thesynthetic resin configuring the outer wheel element, and a thermosettingresin is adopted as the synthetic resin configuring the inner wheelelement).

In a case where the worm wheel of the invention is implemented, forexample, a second annular concave part is provided to be recessed in theaxial direction in the axial other-side surface of the inner wheelelement. The inner wheel element may be embedded in the outer wheelelement, such that a continuous range from the inner diameter sidecircumferential surface configuring the inner surface of the firstannular concave part through the outer circumferential surface of theinner wheel element to an inner diameter side circumferential surfaceconfiguring an inner surface of the second annular concave part in thesurface of the inner wheel element is covered over an entirecircumference.

In a case where the worm wheel of the invention is implemented, forexample, a tilted surface part which is tilted in a direction in which awidth dimension in a radial direction of the second annular concave partbecomes larger toward an axial other side with respect to a central axisof the inner wheel element may be provided in at least onecircumferential surface among an outer diameter side circumferentialsurface and the inner diameter side circumferential surface configuringthe inner surface of the second annular concave part.

In a case where the worm wheel of the invention is implemented, forexample, the tilted surface part and a non-tilted surface part which isnot tilted with respect to the central axis of the inner wheel elementmay be provided in the outer diameter side circumferential surfaceconfiguring the inner surface of the second annular concave part.

In a case where the worm wheel of the invention is implemented, forexample, the tilted surface part and the non-tilted surface part whichis not tilted with respect to the central axis of the inner wheelelement may be provided in the inner diameter side circumferentialsurface configuring the inner surface of the second annular concavepart.

In a case where the worm wheel of the invention is implemented, forexample, in the inner wheel element, a portion which is positioned on aradially outer side from the first annular concave part and the secondannular concave part and a portion which is interposed between a bottomsurface of the first annular concave part and a bottom surface of thesecond annular concave part are each formed such that axial dimensionsof both portions which interpose a central position of the inner wheelelement in the axial direction may be the same as each other. In theouter wheel element, a portion which is positioned on a radially outerside from the inner wheel element and a portion which is superimposed inthe axial direction with respect to the portion which is positioned onthe radially outer side from the first annular concave part and thesecond annular concave part in the inner wheel element are each formedsuch that axial dimensions of both portions which interpose the centralposition of the inner wheel element in the axial direction may be thesame as each other.

In a case where the worm wheel of the invention is implemented, forexample, a concave-convex part in a circumferential direction may beprovided in the surface of the inner wheel element, and a portion of asynthetic resin configuring the outer wheel element may enter into aconcave part configuring the concave-convex part.

In a case where the worm wheel of the invention is implemented, forexample, the concave-convex part may be provided in the inner surface ofthe first annular concave part or the second annular concave part.

In this case, for example, the concave-convex part may be provided inthe outer diameter side circumferential surface or the inner diameterside circumferential surface configuring the inner surface of the firstannular concave part or the second annular concave part.

In this case, for example, the concave-convex part may be provided overthe entire axial length of the outer diameter side circumferentialsurface or the inner diameter side circumferential surface configuringthe inner surface of the first annular concave part or the secondannular concave part, and a portion of the synthetic resin configuringthe outer wheel element may enter into the entire concave partconfiguring the concave-convex part.

In a case where the worm wheel of the invention is implemented, in acase where the concave-convex part is provided in the outer diameterside circumferential surface or the inner diameter side circumferentialsurface configuring the inner surface of the first annular concave partor the second annular concave part, for example, a plurality of concaveparts and a plurality of convex parts configuring the concave-convexpart may be formed in parallel to the axial direction of the worm wheel.

Further, the plurality of teeth configuring the worm wheel tooth partmay be formed in the direction which is tilted in a predetermineddirection with respect to the axial direction of the worm wheel, and theplurality of concave parts and the plurality of convex parts configuringthe concave-convex part may be formed in the direction which is tiltedin the opposite direction to the predetermined direction with respect tothe axial direction of the worm wheel.

In a case where the worm wheel of the invention is implemented, forexample, a sub concave part may be provided to be recessed radiallyoutward in a portion which is positioned on an axial deep side from anaxial opening-side end edges of the first annular concave part in theouter diameter side circumferential surface configuring the innersurface of the first annular concave part, and a portion of thesynthetic resin configuring the outer wheel element may enter into thesub concave part.

In this case, for example, a cross section of the sub concave part withrespect to a virtual plane including the central axis of the inner wheelelement may have a V shape such that a width dimension in the axialdirection becomes smaller from an opening part on an inner diameter sidetoward a bottom part on an outer diameter side.

In a case where the worm wheel of the invention is implemented, forexample, the inner wheel element may be formed to be a cylindricalsurface part in an axial range of at least a portion of the outercircumferential surface.

In a case where the worm wheel of the invention is implemented, in acase where the entire outer circumferential surface of the inner wheelelement serves as the cylindrical surface part, for example, theradially outer end parts of both axial surfaces of the inner wheelelement, which are continuous (directly or through the chamferedportion) with respect to both axial end edges of the cylindrical surfacepart which is the outer circumferential surface of the inner wheelelement each may serve as flat surface parts orthogonal to the centralaxis of the inner wheel element. Accordingly, the both axial end edgesof the cylindrical surface part which is the outer circumferentialsurface of the inner wheel element each can be formed to have a circularshape in which the axial position is not changed in the circumferentialdirection.

In a case where the invention is implemented, for example, in thesurface of the inner wheel element, at least one portion (for example,the cylindrical surface part and the entire surface of the inner wheelelement) in portions which are covered with the synthetic resinconfiguring the outer wheel element may serve as a minute concave-convexsurface formed by various kinds of processes such as a knurling process,an emboss process (a process to transfer the minute concave-convexformed in the surface of hard metal to the surface of the moldedarticle), and a shot blast.

With such a configuration, a portion of the synthetic resin configuringthe outer wheel element enters into the concave part configuring theminute concave-convex surface, and thus it is possible to improve theholding power (adhesiveness) of the outer wheel element with respect tothe inner wheel element.

Incidentally, the depth of the concave part configuring the minuteconcave-convex surface is preferably set to be equal to or less than onetenth (preferably, equal to or less than one twentieth, and morepreferably, equal to or less than one thirtieth) of the radial height ofthe teeth configuring the worm wheel tooth part, so that the volume ofthe synthetic resin configuring the outer wheel element is hardlyaffected.

In a case where the worm wheel of the invention is implemented, forexample, a projecting part which projects on the axial other side from aportion adjacent to the radially outer side may be provided in aradially inner end part (for example, the same radial position as theradially inner end part of the second annular concave part in the axialother-side surface of the outer wheel element) of an axial other-sidesurface of the outer wheel element.

A worm reduction gear of the invention includes a housing, a rotationshaft, a worm wheel, and a worm shaft.

The rotation shaft is supported to be rotatable with respect to thehousing.

Further, the worm wheel has a worm wheel tooth part in an outercircumferential surface and is externally fitted and fixed to therotation shaft.

Further, the worm shaft has a worm tooth part in an axial intermediateportion of an outer circumferential surface and is supported to berotatable with respect to the housing in a state where the worm toothpart is engaged with the worm wheel tooth part.

Particularly, in the case of the worm reduction gear of the invention,the worm wheel is the worm wheel of the invention.

In a case where the worm reduction gear of the invention is implemented,for example, the outer circumferential surface of the inner wheelelement configuring the worm wheel may be formed to be a cylindricalsurface part in an axial range which is radially superimposed with atleast an axial portion (for example, an axial intermediate portion or anaxial end part) of an engaging portion of the worm wheel tooth part andthe worm tooth part.

In this case, for example, the outer circumferential surface of theinner wheel element may be formed to be the cylindrical surface part inan axial range which is radially superimposed with the entire engagingportion.

Further, in this case, for example, the entire outer circumferentialsurface (excluding the chamfered portion in a case where the chamferedportion is provided in the axial end edge part of the outercircumferential surface) of the inner wheel element may serve as thecylindrical surface part.

In a case where the worm reduction gear of the invention is implemented,for example, a rolling bearing which includes an inner ring, an outerring, and a plurality of rolling bodies provided between an outercircumferential surface of the inner ring and an inner circumferentialsurface of the outer ring, and supports the rotation shaft to berotatable with respect to the housing may be provided in a portionadjacent to an axial other side of the worm wheel.

Further, an axial other-side surface of the outer wheel elementconfiguring the worm wheel may face an axial one-side surface of theinner ring and an axial one-side surface of the outer ring in the axialdirection.

Further, an axial distance between the axial other-side surface of theouter wheel element and the axial one-side surface of the inner ring maybe smaller than an axial distance between the axial other-side surfaceof the outer wheel element and the axial one-side surface of the outerring.

In a case where the worm reduction gear of the invention is implemented,for example, a projecting part which projects on the axial other sidefrom a portion adjacent to the radially outer side may be provided in aradially inner end part of the axial other-side surface of the outerwheel element configuring the worm wheel, and an axial other-sidesurface of the projecting part may face the axial one-side surface ofthe inner ring in the axial direction.

To put this in another manner, in the axial other-side surface of theouter wheel element, the projecting part which projects on the axialother side from the portion which faces the axial one-side surface ofthe outer ring in the axial direction may be provided in the portionwhich faces the axial one-side surface of the inner ring in the axialdirection.

A manufacturing method of the worm wheel of the invention is amanufacturing method for the worm wheel of the invention.

In the manufacturing method of the worm wheel of the invention, aradially outer end part of a disc gate may be positioned in the radiallyinner end part of the axial other side of the outer wheel element whenan insertion molding is performed in which the outer wheel element iscoupled with the inner wheel element at the same time when the outerwheel element is manufactured by the injection molding.

Advantages of the Invention

With the worm wheel, the worm reduction gear, and the manufacturingmethod of the worm wheel according to the above-described invention, itis possible to improve the holding power of the synthetic resin outerwheel element with respect to the inner wheel element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view similar with FIG. 22 according to a firstembodiment of the invention.

FIG. 2 is a sectional view of a worm wheel according to the firstembodiment.

FIG. 3 is a sectional view partially illustrating a state where a wormtooth part is engaged with a worm wheel tooth part according to thefirst embodiment.

FIG. 4 is a half-sectional view of the worm wheel according to the firstembodiment when partially cut away.

FIG. 5 is a sectional view taken along line A-A of FIG. 2 according tothe first embodiment.

FIG. 6 is an enlarged view of B portion of FIG. 5 according to the firstembodiment.

FIG. 7 is a sectional view illustrating a state where an outer wheelelement is injection-molded according to the first embodiment.

FIG. 8 is a sectional view of a worm wheel according to a secondembodiment of the invention.

FIG. 9 is a sectional view of a worm wheel according to a thirdembodiment of the invention.

FIG. 10 is a half-sectional view of a worm wheel according to a fourthembodiment of the invention when partially cut away.

FIG. 11 is a sectional view of a worm wheel according to a fifthembodiment of the invention.

FIG. 12 is a half-sectional view of a worm wheel according to a sixthembodiment of the invention.

FIG. 13 is a sectional view similar with FIG. 5 according to a seventhembodiment of the invention.

FIGS. 14A and 14B are views of a cut-away half portion of a worm wheelaccording to the seventh embodiment when viewed from a radial direction.

FIG. 15 is a view of an inner wheel element according to the seventhembodiment when viewed from an axial one side.

FIG. 16 is an enlarged view of C portion of FIG. 13 according to theseventh embodiment.

FIG. 17 is a view similar with FIG. 3 according to an eighth embodimentof the invention.

FIG. 18 is a sectional view of a worm wheel according to a ninthembodiment of the invention.

FIG. 19 is a sectional view illustrating a state where an outer wheelelement according to the ninth embodiment is injection-molded.

FIG. 20 is a side view illustrating one example of a structure of theelectric power steering device in the related art partially cut away.

FIG. 21 is an enlarged sectional view taken along line D-D of FIG. 20.

FIG. 22 is an enlarged sectional view taken along line E-E of FIG. 20.

FIG. 23 is a sectional view of the worm wheel.

FIG. 24 is a sectional view taken along line F-F of FIG. 23.

FIG. 25 is an enlarged view of G portion of FIG. 24.

MODES FOR CARRYING OUT THE INVENTION First Embodiment

A first embodiment of the invention will be described with reference toFIGS. 1 to 7.

FIG. 1 illustrates an electric power steering device in which a wormreduction gear of this embodiment is assembled. A front end part of asteering shaft 2 in which a steering wheel 1 (see FIG. 20) is attachedto a rear end part is supported to be rotatable in a housing 3. A wormwheel 4 a is fixed to a portion rotationally driven by the steeringshaft 2. On the other hand, a worm shaft 6 (see FIGS. 3 and 21) isconnected to an output shaft of an electric motor 5. Further, a wormtooth part 18 provided in the outer circumferential surface of the axialintermediate portion of the worm shaft 6 is engaged with a worm wheeltooth part 19 a provided in the outer circumferential surface of theworm wheel 4 a, so that a predetermined magnitude of auxiliary torque(auxiliary power) can be applied from the electric motor 5 with respectto the worm wheel 4 a in the predetermined direction.

The worm wheel 4 a is externally fitted and fixed to an axialintermediate portion of an output shaft 7 which corresponds to arotation shaft described in appended claims and serves as an output partof the auxiliary torque, and rotates together with the output shaft 7.The output shaft 7 is coupled with the front end part of the steeringshaft 2 through a torsion bar 9 in the housing 3 in a state where aportion near both ends of the axial intermediate portion is supported tobe rotatable by one pair of rolling bearings 8 a and 8 b. The electricmotor 5 rotationally drives the worm shaft 6 according to a directionand a magnitude of a steering torque which is detected by a torquesensor 10 and is applied from the steering wheel 1 to the steering shaft2, and the auxiliary torque is applied to the output shaft 7. Therotation of the output shaft 7 is transmitted to a pinion shaft 14 (seeFIG. 20) which serves as an input part of a steering gear unit 13through one pair of universal joints 11 a and 11 b and an intermediateshaft 12, and a desired steering angle is given to a steering wheel.

Incidentally, in the case of the illustrated example, one pair of therolling bearings 8 a and 8 b are ball bearings which include inner rings33 a and 33 b which are externally fitted to and supported by the outputshaft 7, outer rings 34 a and 34 b internally fitted to and supported bythe housing 3, and a plurality of balls 35 a and 35 b which each are arolling body and are provided to be rollable between an inner ringraceway provided in the outer circumferential surface of the inner rings33 a and 33 b and an outer ring raceway provided in the innercircumferential surface of the outer rings 34 a and 34 b, respectively.Herein, in a case where the invention is implemented, another kind ofrolling bearing such as a cylindrical roller bearing or a tapered rollerbearing also may be adopted as one pair of rolling bearings 8 a and 8 b.

The worm wheel 4 a is formed by combining an inner wheel element 15 aand an outer wheel element 16 a.

The inner wheel element 15 a is formed of metal in an annular shape(substantially circular ring shape), and a fitting hole 21 forinternally fitting and fixing the axial intermediate portion of theoutput shaft 7 in a torque-transmittable manner is included in a radialintermediate portion.

A first annular concave part 22 is provided to be recessed in an axialdirection over the entire circumference of a radial intermediate portionof an axial one-side surface (the left surface in FIGS. 1 to 4 and 7) ofthe inner wheel element 15 a.

In an inner diameter side circumferential surface, an outer diameterside circumferential surface, and a bottom surface (axial one-sidesurface) configuring the inner surface of the first annular concave part22, the outer diameter side circumferential surface is provided with a(gear-shaped) concave-convex part 23 in the circumferential directionwhich is formed in such a manner that a concave part 26 and a convexpart 27 are arranged alternately (to have constant pitch in theillustrated example) in the circumferential direction in the entirelength and the entire circumference of the outer diameter sidecircumferential surface in an axial direction. In the case of thisembodiment, as illustrated by a broken line (hidden line) in an upperhalf portion in FIG. 4, a plurality of the concave parts 26 and aplurality of the convex parts 27 configuring the concave-convex part 23are formed in parallel to the axial direction (a right and leftdirection in FIGS. 1 to 4 and 7) of the inner wheel element 15 a (wormwheel 4 a). To put this in another manner, the boundary between theconcave part 26 and the convex part 27 is formed in parallel to theaxial direction of the worm wheel 4 a.

Further, the inner diameter side circumferential surface whichconfigures the inner surface of the first annular concave part 22 servesas a cylindrical surface part 36 having a simple cylindrical surfaceshape.

Further, the bottom surface which configures the inner surface of thefirst annular concave part 22 serves as an intermediate flat surfacepart 37 a which is orthogonal to a central axis of the inner wheelelement 15 a and has a circular ring shape.

A second annular concave part 38 is provided to be recessed in the axialdirection over the entire circumference of the radial intermediateportion of the axial other-side surface (the right surface in FIGS. 1 to4 and 7) of the inner wheel element 15 a.

In the inner diameter side circumferential surface, the outer diameterside circumferential surface, and the bottom surface (axial other-sidesurface) configuring the inner surface of the second annular concavepart 38, the outer diameter side circumferential surface is formed suchthat respective axial end edges of an outer diameter side tilted surfacepart 39 provided in one side (an one side portion and an intermediateportion) and an outer diameter side cylindrical surface part 40 providedin the other side (other end portion) are continuous to each other inthe axial direction. The outer diameter side tilted surface part 39corresponds to a tilted surface part described in appended claims, andis formed in a partially tapered surface shape that is tilted in adirection (the direction toward the radially outer side) in which aradial width dimension of the second annular concave part 38 is largertoward the axial other side with respect to the central axis of theinner wheel element 15 a. With respect thereto, the outer diameter sidecylindrical surface part 40 corresponds to a non-tilted surface partdescribed in appended claims, and is formed in a simple cylindricalsurface shape not to be tilted with respect to the central axis of theinner wheel element 15 a.

Further, the inner diameter side circumferential surface configuring theinner surface of the second annular concave part 38 is configured onlyby an inner diameter side tilted surface part 41. The inner diameterside tilted surface part 41 corresponds to the tilted surface partdescribed in appended claims, and is formed in a partially taperedsurface shape that is tilted in a direction (the direction toward theradially inner side) in which the radial width dimension of the secondannular concave part 38 is larger toward the axial other side withrespect to the central axis of the inner wheel element 15 a.

Further, the bottom surface configuring the inner surface of the secondannular concave part 38 serves as an intermediate flat surface part 37 bof the circular ring shape which is orthogonal to the central axis ofthe inner wheel element 15 a.

In the inner wheel element 15 a, the entire outer circumferentialsurface (excluding chamfered portions thereof in a case where thechamfered portions are provided in both axial end edge parts) serves asa cylindrical surface part 24 in which a radial distance from thecentral axis of the inner wheel element 15 a is not substantiallychanged over the entire circumference. In the case of this embodiment,the cylindrical surface part 24 is formed in a simple cylindricalsurface shape which has a bus line parallel to the central axis of theinner wheel element 15 a (worm wheel 4 a) and has a diameter which isnot changed in the axial direction. Further, the radially outer end part(the portion positioned on the radially outer side from the firstannular concave part 22) of the axial one-side surface of the innerwheel element 15 a and the radially outer end part (the portionpositioned on the radially outer side from the second annular concavepart 38) of the axial other-side surface of the inner wheel element 15 aare continuous (directly or through the chamfered portion) with respectto both axial end edges of the cylindrical surface part 24 and serve asflat surface parts 25 a and 25 b which have a circular ring shape andare orthogonal to the central axis of the inner wheel element 15 a,respectively.

To put this in another manner, in the case of this embodiment, the innerwheel element 15 a includes an inner diameter side annular part 28 andan outer diameter side annular part 29 which are arranged coaxially witheach other, and a connection part 30 which has a circular ring shape andconnects the axial intermediate portions of the outer circumferentialsurface of the inner diameter side annular part 28 and the innercircumferential surface of the outer diameter side annular part 29.Further, the outer circumferential surface of the outer diameter sideannular part 29 serves as the cylindrical surface part 24, and bothaxial surfaces of the outer diameter side annular part 29 serve as theflat surface parts 25 a and 25 b. Further, the portion surrounded bythree sides of the axial one end portion of the outer circumferentialsurface of the inner diameter side annular part 28, the axial one endportion of the inner circumferential surface of the outer diameter sideannular part 29, and the axial one-side surface of the connection part30 serves as the first annular concave part 22. The portion surroundedby three sides of the axial other end portion of the outercircumferential surface of the inner diameter side annular part 28, theaxial other end portion of the inner circumferential surface of theouter diameter side annular part 29, and the axial other-side surface ofthe connection part 30 serves as the second annular concave part 38.Further, in the inner circumferential surface of the outer diameter sideannular part 29, the axial one end portion serves as the concave-convexpart 23, and the axial other end portion serves as the outer diameterside tilted surface part 39 and the outer diameter side cylindricalsurface part 40. Further, in the outer circumferential surface of theinner diameter side annular part 28, the axial one end portion serves asthe cylindrical surface part 36, and the axial other end portion servesas the inner diameter side tilted surface part 41.

Incidentally, various kinds of metals such as copper alloy, aluminumalloy, and magnesium alloy in addition to ferrous metal such as iron andsteel may be adopted as a metal configuring the inner wheel element 15a. Further, various kinds of cutting processes or plastic processes canbe adopted as a process to mold the inner wheel element 15 a. Herein,the plastic process (forging, pressing, flow forming, and the like) ispreferably adopted when molded in a high yield at low cost.

On the other hand, since the outer wheel element 16 a is manufactured byinjection-molding a synthetic resin, a portion of the inner wheelelement 15 a is embedded over the entire circumference through theinjection molding (insertion molding). Specifically, one portion (theradially outer end part and the intermediate portion) of the inner wheelelement 15 a is embedded in the outer wheel element 16 a, such that thecontinuous range from the inner diameter side circumferential surface(cylindrical surface part 36) configuring the inner surface of the firstannular concave part 22, through the outer circumferential surface(cylindrical surface part 24) of the inner wheel element 15 a, to theinner diameter side circumferential surface (inner diameter side tiltedsurface part 41) configuring the inner surface of the second annularconcave part 38 in the surface of the inner wheel element 15 a iscovered over the entire circumference. In this state, a portion of thesynthetic resin enters into the first annular concave part 22 and thesecond annular concave part 38. Thus, the portion entering into thefirst annular concave part 22 configures a first suppression part 31having an annular shape and the portion entering into the second annularconcave part 38 configures a second suppression part 42 having anannular shape. Further, in this state, a portion of the synthetic resinconfiguring the first suppression part 31 enters into the entire portionof the plurality of concave parts 26 and 26 configuring theconcave-convex part 23 (the portion between the convex parts 27 and 27adjacent in the circumferential direction), so as to cover the entiresurface of the concave-convex part 23. Thus, a rotation holding part 32is configured to be engaged with the concave-convex part 23 (have ashape coinciding with the concave-convex part 23). Further, thethickness (excluding the thickness of the portion entering into theplurality of concave parts 26 and 26) of the first suppression part 31is wholly almost the same, and the thickness of the second suppressionpart 42 also is wholly almost the same. Further, in the axial other-sidesurface of the outer wheel element 16 a, the projecting part 43 isprovided over the entire circumference in a radial position which is thesame as the radially inner end part of the second annular concave part38 in the state of projecting from the inside (the axial other-sidesurface of the second suppression part 42) of the second annular concavepart 38 to the axial other side.

The worm wheel tooth part 19 a is formed in the outer circumferentialsurface of the outer wheel element 16 a. The axial intermediate portionof the worm wheel tooth part 19 a is radially superimposed with thecylindrical surface part 24. Further, as illustrated in the upper halfportion of FIG. 4, the direction of forming the plurality of teethconfiguring the worm wheel tooth part 19 a is tilted with respect to theaxial direction of the worm wheel 4 a. Further, in the case of thisembodiment, a tooth tip circle diameter and a tooth bottom circlediameter of the worm wheel tooth part 19 a are not changed with respectto the axial direction, respectively.

In the case of this embodiment, at the same time when the outer wheelelement 16 a is manufactured by the injection molding, when theinsertion molding is performed in which the outer wheel element 16 a iscoupled with respect to the inner wheel element 15 a, as illustrated inFIG. 7, in a state where the inner wheel element 15 a is set in amolding device 44 formed by combining a plurality of dies, molten resinis fed into an annular cavity 45 which is defined between the innerwheel element 15 a and the molding device 44 from a runner 46 and a discgate 47 which are provided in the axial other-side surface side of theinner wheel element 15 a. In the cavity 45, the radially outer end part(outer circumferential part) of the disc gate 47 is positioned in aportion in which the radially inner end part (projecting part 43) of theaxial other side of the outer wheel element 16 a is formed, and therunner 46 is provided to extend from the central portion of the discgate 47 to the axial other side. The molten resin fed into the cavity 45from the runner 46 and the disc gate 47 flows from the inner diameterside to the outer diameter side along the axial other-side surface ofthe inner wheel element 15 a, and part thereof enters into the secondannular concave part 38. At that time, in the case of this embodiment,the inner diameter side tilted surface part 41 and the outer diameterside tilted surface part 39 are provided in the inner diameter sidecircumferential surface and the outer diameter side circumferentialsurface configuring the inner surface of the second annular concave part38. Thus, without large disturbance to the flow, the molten resin entersinto the portion in which the radially outer end part of the outer wheelelement 16 a in the cavity 45 is formed. Further, the molten resin fedinto the cavity 45 reaches a portion corresponding to the firstsuppression part 31 and stops. The portion does not abut on the moltenresin having flowed from the other direction. As a result, it can beprevented that weld or the like which is weak in strength occurs in theouter wheel element 16 a obtained by the injection molding. The moldingdevice 44 is opened and the plurality of dies are separated from eachother. Then, the synthetic resin which is cooled and solidified in thecavity 45 is cut over the entire circumference in the portioncorresponding to the radially inner end part of the axial other side ofthe outer wheel element 16 a, and the worm wheel 4 a is obtained byperforming a finishing process as needed. Incidentally, at that time, inthe case of this embodiment, a portion (the portion to be removedoriginally) of the synthetic resin positioned in the radially outer endpart of the disc gate 47 remains without removal, to form the projectingpart 43. Accordingly, a yield is excellent, and the projecting part 43can be formed at low cost.

Incidentally, in addition to a polyamide 66 (PA66), various kinds ofsynthetic resins such as the polyamide 46 (PA46), a polyamide 9T (PA9T),a polyphenylene sulfide (PPS), a polyethylene terephthalate (PET), and apolyacetal (POM) may be adopted as a synthetic resin configuring theouter wheel element 16 a. Further, as needed, various kinds ofreinforced fibers such as a glass fiber, a polyethylene fiber, a carbonfiber, and an aramid fiber may be mixed in the synthetic resin.

Further, in a state where the electric power steering device of thisembodiment is assembled, at least an axial portion in the engagingportion 50 (the portion indicated by cross-hatched pattern in FIG. 3) ofthe worm tooth part 18 and the worm wheel tooth part 19 a is configuredto be radially superimposed with the cylindrical surface part 24provided in the outer circumferential surface of the inner wheel element15 a.

Particularly, in the case of this embodiment, the entire engagingportion 50 is configured to be radially superimposed with thecylindrical surface part 24. For this reason, the axial width dimensionS of the engaging portion 50 is set to be equal to or less than an axialwidth dimension T (S≤T(S<T in the example illustrated in FIG. 3)) of thecylindrical surface part 24. The axial range positioned in the engagingportion 50 is within the axial range positioned in the cylindricalsurface part 24.

Herein, in a case where the invention is implemented, for example, theaxial width dimension S of the engaging portion 50 is set to be largerthan the axial width dimension T of the cylindrical surface part 24(S>T), and the axial range positioned in the cylindrical surface part 24may be configured to be within the axial range positioned in theengaging portion 50.

In a state where the electric power steering device of this embodimentis assembled, the axial other-side surface of the outer wheel element 16a and the axial one-side surface of the inner ring 33 a and the axialone-side surface of the outer ring 34 a which are provided in theportion adjacent to the axial other side of the worm wheel 4 a andconfigure the rolling bearing 8 a, face each other in the axialdirection through a gap. Specifically, in the axial other-side surfaceof the outer wheel element 16 a, the portion corresponding to theprojecting part 43 faces the axial one-side surface of the inner ring 33a in the axial direction, and the portion corresponding to the radialintermediate portion of the second suppression part 42 faces the axialone-side surface of the outer ring 34 a in the axial direction. Herein,in the case of this embodiment, the axial one-side surface of the innerring 33 a and the axial one-side surface of the outer ring 34 a areprovided in almost the same position in the axial direction while in theaxial other-side surface of the outer wheel element 16 a, the portion(the axial other-side surface of the projecting part 43) correspondingto the projecting part 43 is positioned on the axial other side (by atleast an axial dimension of the projecting part 43) from the portion(the axial other-side surface of the radial intermediate portion of thesecond suppression part 42) corresponding to the radial intermediateportion of the second suppression part 42. For this reason, an axialdistance X between the axial other-side surface (the axial other-sidesurface of the projecting part 43) of the outer wheel element 16 a andthe axial one-side surface of the inner ring 33 a is smaller than theaxial distance Y between the axial other-side surface (the axialother-side surface of the radial intermediate portion of the secondsuppression part 42) of the outer wheel element 16 a and the axialone-side surface of the outer ring 34 a (X<Y).

In the worm wheel 4 a and the worm reduction gear of this embodimentconfigured as above, the holding power of the synthetic resin outerwheel element 16 a can be secured from the metal inner wheel element 15a, and the manufacturing error can be suppressed in the worm wheel toothpart 19 a provided in the outer circumferential surface of the outerwheel element 16 a. The description will be given below about thesepoints.

When the auxiliary torque is applied to the output shaft 7 through theworm wheel 4 a, the moment M in a tilted direction is applied to theworm wheel 4 a as illustrated by an arrow in FIG. 2 on the basis of anaxial component of an engaging counterforce which acts on the engagingportion of the worm wheel tooth part 19 a of the worm wheel 4 a and theworm tooth part 18 of the worm shaft 6.

With respect thereto, in the case of this embodiment, the first annularconcave part 22 and the second annular concave part 38 are provided overthe entire circumferences in both axial surfaces of the inner wheelelement 15 a, respectively. Further, a portion of the inner wheelelement 15 a is embedded in the outer wheel element 16 a, such that therange which is continuous from the inner diameter side circumferentialsurface (cylindrical surface part 36) configuring the inner surface ofthe first annular concave part 22 through the outer circumferentialsurface (cylindrical surface part 24) of the inner wheel element 15 a tothe inner diameter side circumferential surface (inner diameter sidetilted surface part 41) configuring the inner surface of the secondannular concave part 38 in the surface of the inner wheel element 15 ais covered over the entire circumference. That is, in the case of thisembodiment, the radial intermediate portion in the inner wheel element15 a as well as the radially outer end part is embedded in the outerwheel element 16 a. For this reason, the holding power of the outerwheel element 16 a in the direction of the moment M with respect to theinner wheel element 15 a can be larger compared to a case where only theradially outer end part is embedded.

Further, in the case of this embodiment, a portion of the syntheticresin configuring the outer wheel element 16 a enters into the firstannular concave part 22 to configure the first suppression part 31having an annular shape, and enters into the second annular concave part38 to configure the second suppression part 42 having an annular shape.For this reason, the holding power of the outer wheel element 16 a inthe direction of the moment M with respect to the inner wheel element 15a can be enlarged on the basis of the engagement of the first annularconcave part 22 and the first suppression part 31 and the engagement ofthe second annular concave part 38 and the second suppression part 42.

In the case of this embodiment, the outer diameter side cylindricalsurface part 40 is provided in the axial other end portion of the outerdiameter side circumferential surface configuring the inner surface ofthe second annular concave part 38. For this reason, the engaging powerof the second annular concave part 38 and the second suppression part 42with respect to the moment M can be enlarged compared to a case wherethe entire outer diameter side circumferential surface serves as theouter diameter side tilted surface part 39 (the outer diameter sidecylindrical surface part 40 is not provided). As a result, it ispossible to enlarge the holding power of the outer wheel element 16 a inthe direction of the moment M with respect to the inner wheel element 15a.

In the case of this embodiment, the concave-convex part 23 in thecircumferential direction is provided in the outer diameter sidecircumferential surface configuring the inner surface of the firstannular concave part 22, and a portion of the synthetic resinconfiguring the first suppression part 31 enters into the plurality ofthe entire concave parts 26 and 26 configuring the concave-convex part23 to cover the entire surface of the concave-convex part 23, therebyconfiguring the rotation holding part 32 which is engaged with theconcave-convex part 23 (has a shape coinciding with the concave-convexpart 23). For this reason, in the case of this embodiment. it ispossible to secure the holding power of the outer wheel element 16 a inthe rotation direction with respect to the inner wheel element 15 a.Particularly, in the case of this embodiment, the concave-convex part 23is provided over the entire axial length of the outer diameter sidecircumferential surface configuring the inner surface of the firstannular concave part 22, and thus the holding power in the rotationdirection can be large.

The outer circumferential surface of the inner wheel element 15 a servesas the cylindrical surface part 24 in which the diameter in the axialdirection is not changed. For this reason, in the outer wheel element 16a, the portion which is superimposed on the radially outer side withrespect to the cylindrical surface part 24 which is the outercircumferential surface of the inner wheel element 15 a has the(substantially) same radial thickness in the portions where theplurality of teeth 20 a and 20 a configuring the worm wheel tooth part19 a are positioned.

The tooth tip circle diameter and the tooth bottom circle diameter ofthe worm wheel tooth part 19 a provided in the outer circumferentialsurface of the outer wheel element 16 a is not changed in the axialdirection, respectively. Together with this, the radially outer endparts of the both axial surfaces of the inner wheel element 15 a whichare continuous with respect to the both axial end edges of thecylindrical surface part 24 which is the outer circumferential surfaceof the inner wheel element 15 a serve as the flat surface parts 25 a and25 b which have the circular ring shape and are orthogonal to thecentral axis of the inner wheel element 15 a, respectively. In otherwords, the both axial end edges of the cylindrical surface part 24 whichis the outer circumferential surface of the inner wheel element 15 aeach have a circular shape in which the axial position is not changed inthe circumferential direction. For this reason, in the case of thisembodiment, in the outer wheel element 16 a, the portion which issuperimposed on the radially outer side with respect to the outercircumferential surface of the inner wheel element 15 a has almost thesame radial thickness over the entire axial length including both axialend edge parts in the portions in which the plurality of teeth 20 a and20 a configuring the worm wheel tooth part 19 a are positioned.

Accordingly, in the outer wheel element 16 a, as illustrated in FIG. 6,the portion which is superimposed on the radially outer side withrespect to at least the outer circumferential surface of the inner wheelelement 15 a can be set such that the amount of the molding shrinkage,which occurs during the injection molding, of the portions in which theplurality of teeth 20 a and 20 a are positioned is almost the same. As aresult, the plurality of teeth 20 a and 20 a after molding can be set tohave almost the same magnitude (radial thickness), so as to suppress themanufacturing error such as the pitch error with respect to the wormwheel tooth part 19 a.

The plurality of concave parts 26 and the plurality of the convex parts27 configuring the concave-convex part 23 are formed in parallel to theaxial direction. For this reason, it is suppressed that theconcave-convex part 23 causes the deformation of the outer wheel element16 a associated with the molding shrinkage of the synthetic resin, andthe outer wheel element 16 a can be molded accurately.

In a state where the worm reduction gear is assembled, the entireengaging portion 50 of the worm tooth part 18 and the worm wheel toothpart 19 a is radially superimposed with the cylindrical surface part 24.In other words, in the worm wheel tooth part 19 a, the worm tooth part18 is engaged with the portion which suppresses the manufacturing errorsuch as the pitch error as described above. For this reason, it ispossible to make the engaged state of the engaging portion 50 excellent.Incidentally, in a case where the invention is implemented, in a casewhere only the axial portion of the engaging portion 50 is radiallysuperimposed with the cylindrical surface part 24, the engaged state ofthe engaging portion becomes more excellent as a ratio (axial range) tobe superimposed becomes larger.

In order that the entire engaging portion 50 is radially superimposedwith the cylindrical surface part 24, the axial width dimension S of theengaging portion 50 is equal to or less than the axial width dimension Tof the cylindrical surface part 24 (S T). However, under the condition,if the dimensions S and T are set to be almost the same, in a statewhere the axial dimension of the inner wheel element 15 a is suppressedto a minimum extent, the engaged state of the engaging portion 50 can bemade excellent. Incidentally, such a point is also applied to a casewhere the condition of S>T is adopted.

The axial distance X between the axial other-side surface (the axialother-side surface of the projecting part 43) of the outer wheel element16 a and the axial one-side surface of the inner ring 33 a becomessmaller than the axial distance Y between the axial other-side surface(the axial other-side surface of the radial intermediate portion of thesecond suppression part 42) of the outer wheel element 16 a and theaxial one-side surface of the outer ring 34 a (X<Y). For this reason,for example, although a portion which regulates the axial position ofthe output shaft 7 with respect to the housing 3 is broken so that theoutput shaft 7 and the worm wheel 4 a are displaced to the axial otherside, the axial other-side surface of the outer wheel element 16 acontacts only the axial one-side surface (the axial other-side surfaceof the projecting part 43) of the inner ring 33 a among the axialone-side surface of the inner ring 33 a and the axial one-side surfaceof the outer ring 34 a, and does not contact the axial one-side surface(the axial other-side surface of the radial intermediate portion of thesecond suppression part 42) of the outer ring 34 a. Thus, it can beprevented that the rotation of the worm wheel 4 a is locked.

Second Embodiment

A second embodiment of the invention will be described with reference toFIG. 8.

In the case of this embodiment, a second annular concave part 38 aprovided in an axial other-side surface (the right surface in FIG. 8) ofan inner wheel element 15 b configuring a worm wheel 4 b is configuredto be different from the case of the above-described first embodiment.In the case of this embodiment, the inner diameter side circumferentialsurface configuring the inner surface of the second annular concave part38 a is formed such that respective axial end edges of an inner diameterside cylindrical surface part 48 which is provided in a half portion onthe axial one side (left side in FIG. 8) and corresponds to thenon-tilted surface part described in appended claims and the innerdiameter side tilted surface part 41 provided in a half portion of theaxial other side are continuous to each other.

In the case of this embodiment having such a configuration, the innerdiameter side cylindrical surface part 48 is provided in the axial halfportion of the inner diameter side circumferential surface configuringthe inner surface of the second annular concave part 38 a. Thus, theengaging power of the second annular concave part 38 a and a secondsuppression part 42 a configuring an outer wheel element 16 b withrespect to the moment M can be enlarged compared to a case where theentire inner diameter side circumferential surface serves as the innerdiameter side tilted surface part 41 (the inner diameter sidecylindrical surface part 48 is not provided). As a result, it ispossible to enlarge the holding power of the outer wheel element 16 b inthe direction of the moment M with respect to the inner wheel element 15b.

The other configuration and effect are similar with the case of theabove-described first embodiment.

Third Embodiment

A third embodiment of the invention will be described with reference toFIG. 9.

In the case of this embodiment, a second annular concave part 38 bprovided in an axial other-side surface (the right surface in FIG. 9) ofan inner wheel element 15 c configuring a worm wheel 4 c is configuredto be different from the case of the above-described first embodiment.In the case of this embodiment, the inner diameter side circumferentialsurface configuring the inner surface of the second annular concave part38 b is formed such that the inner diameter side tilted surface part 41provided in a half portion on the axial one side (left side in FIG. 9)and an inner diameter side cylindrical surface part 48 a which isprovided in a half portion of the axial other side and corresponds tothe non-tilted surface part described in appended claims are continuousthrough a stepped surface 49 which has a circular ring shape and isdirected to the axial other side.

In the case of this embodiment having such a configuration, the innerdiameter side cylindrical surface part 48 a is provided in the axialother half portion of the inner diameter side circumferential surfaceconfiguring the inner surface of the second annular concave part 38 b.Thus, the engaging power of the second annular concave part 38 b and asecond suppression part 42 b configuring an outer wheel element 16 cwith respect to the moment M can be enlarged compared to a case wherethe entire inner diameter side circumferential surface serves as theinner diameter side tilted surface part 41 (the inner diameter sidecylindrical surface part 48 a is not provided). As a result, it ispossible to enlarge the holding power of the outer wheel element 16 c inthe direction of the moment M with respect to the inner wheel element 15c.

The other configuration and effect are similar with the case of theabove-described first embodiment.

Fourth Embodiment

A fourth embodiment of the invention will be described with reference toFIG. 10.

In the case of this embodiment, a first annular concave part 22 aprovided in an axial one-side surface (the left surface in FIG. 10) ofan inner wheel element 15 d configuring a worm wheel 4 d is configuredto be different from the case of the above-described first embodiment.In the case of this embodiment, in the inner surface of the firstannular concave part 22 a, a plurality of concave parts 26 a and aplurality of convex parts 27 a configuring the concave-convex part 23 aprovided in the outer diameter side circumferential surface are formedto be tilted in an inverse direction to the tilted direction of theplurality of teeth 20 a and 20 a configuring the worm wheel tooth part19 a provided in the outer circumferential surface of the outer wheelelement 16 d with respect to the central axis of the worm wheel 4 d.

In the case of this embodiment, the tilted direction of the plurality ofteeth 20 a and 20 a configuring the worm wheel tooth part 19 a and thetilted direction of the plurality of concave parts 26 a and theplurality of convex parts 27 a configuring the concave-convex part 23 aare set to be opposite to each other with respect to the central axis ofthe worm wheel 4 d. Thus, when the torque is transmitted from the wormshaft 6 (see FIGS. 3 and 21) to the worm wheel 4 d, a force is appliedin a direction in which the synthetic resin configuring the outer wheelelement 16 d does not come out from the plurality of concave parts 26 aand 26 a configuring the concave-convex part 23 a toward the axial oneside.

The other configuration and effect are similar with the case of theabove-described first embodiment.

Fifth Embodiment

A fifth embodiment of the invention will be described with reference toFIG. 11. The case of this embodiment is a modification of the secondembodiment illustrated in FIG. 8.

In the case of this embodiment, an (gear-shaped) concave-convex part 51in the circumferential direction which is formed by alternatelyarranging the concave part 52 and the convex part 53 in thecircumferential direction is provided in the inner diameter sidecircumferential surface configuring the first annular concave part 22 bprovided in an axial one-side surface (the left surface in FIG. 11) ofan inner wheel element 15 e configuring a worm wheel 4 e. Further, an(gear-shaped) concave-convex part 54 in the circumferential directionwhich is formed by alternately arranging the concave part 55 and theconvex part 56 in the circumferential direction is provided in the innerdiameter side circumferential surface configuring the inner surface ofthe second annular concave part 38 c provided in the axial other-sidesurface (the right surface in FIG. 11) of the inner wheel element 15 e.Further, the synthetic resin configuring the outer wheel element 16 eenters into the concave parts 52 and 52 configuring the concave-convexpart 51 and the concave parts 55 and 55 configuring the concave-convexpart 54. Further, the holding power of the outer wheel element 16 e inthe rotation direction with respect to the inner wheel element 15 e isimproved on the basis of the engagement of both concave-convex parts 51and 54 and the synthetic resin.

The other configuration and effect are similar with the case of theabove-described second embodiment.

Sixth Embodiment

A sixth embodiment of the invention will be described with reference toFIG. 12. The case of this embodiment is a modification of the firstembodiment illustrated in FIGS. 1 to 7.

In the case of this embodiment, the portion (outer diameter side annularpart 29) positioned on the radially outer side from both first andsecond annular concave parts 22 and 38 in an inner wheel element 15 fconfiguring a worm wheel 4 f, and the portion (connection part 30) whichis interposed between the bottom surfaces (intermediate flat surfaceparts 37 a and 37 b) configuring the inner surfaces of the both firstand second annular concave parts 22 and 38 in the inner wheel element 15f are each formed such that the axial dimensions of the both portionswhich interpose a central position Ca of the inner wheel element 15 f inthe axial direction are the same as each other (as L_(A) or L_(B)).

The portion (including the worm wheel tooth part 19 a) positioned on theradially outer side from the inner wheel element 15 f in an outer wheelelement 16 f configuring the worm wheel 4 f, and the portion (theportion positioned on both axial sides of the outer diameter sideannular part 29) superimposed in the axial direction with respect to theportion (outer diameter side annular part 29) positioned on the radiallyouter side from the both first and second annular concave parts 22 and38 of the inner wheel element 15 f in the outer wheel element 16 f areeach formed such that the axial dimensions of the both portions whichinterpose the central position Ca in the axial direction are the same aseach other (as L_(C) or L_(D)).

In the case of this embodiment having the above-described configuration,when the inner wheel element 15 f and the outer wheel element 16 fconfiguring the worm wheel 4 f is expanded thermally (contractedthermally) on the basis of the temperature change, in theabove-described portions, the axial dimensions L_(A), L_(B), L_(C), andL_(D) of both portions which interpose the central position Ca in theaxial direction are each changed equally. For this reason, it can beprevented effectively that the engagement of the worm wheel tooth part19 a and the worm tooth part 18 (see FIG. 3) is deviated according tothe temperature change.

The other configuration and effect are similar with the case of theabove-described first embodiment.

Seventh Embodiment

A seventh embodiment of the invention will be described with referenceto FIGS. 13 to 16. This embodiment is a modification of theabove-described first embodiment illustrated in FIGS. 1 to 7.

In the case of this embodiment, a surface of a cylindrical surface part24 a provided in an outer circumferential surface of an inner wheelelement 15 g configuring a worm wheel 4 g serves as a knurling surface57 which is a minute concave-convex surface formed by a knurlingprocess. In the case of this embodiment, a flat-pattern surface in whicha direction in which the concave-convex is formed is in parallel to theaxial direction as illustrated in the upper half portion of FIG. 14A isadopted as the knurling surface 57. However, in a case where theinvention is implemented, for example, a twill-pattern surface in whicha direction in which the concave-convex is formed is tilted to andintersected with the axial direction as illustrated in the upper halfportion of FIG. 14B may be adopted.

In any case, in the case of this embodiment, radial depth dimensionsW_(58a) and W_(58b) of minute concave parts 58 a and 58 b configuringthe knurling surface 57 are set to be equal to or less than one tenth(W_(58a)≤H_(20a)/10 and W_(58b)≤H_(20a)/10) of the radial heightdimension (tooth depth) H_(20a) of the teeth 20 a configuring the wormwheel tooth part 19 a provided in the outer circumferential surface ofthe outer wheel element 16 g.

In the case of this embodiment, a portion of the synthetic resinconfiguring the outer wheel element 16 g enters into the minute concaveparts 58 a and 58 b configuring the knurling surface 57.

In the case of this embodiment having the above-described configuration,the holding power of the outer wheel element 16 g in the rotationdirection with respect to the inner wheel element 15 g can be improvedon the basis of the engagement of a portion of the synthetic resinconfiguring the outer wheel element 16 g and the minute concave parts 58a and 58 b configuring the knurling surface 57. Incidentally, if atwill-pattern surface is adopted as the knurling surface 57, the holdingpower in the direction of the moment M as well as the holding power ofthe outer wheel element 16 g in the rotation direction with respect tothe inner wheel element 15 g can be improved on the basis of theengagement of a portion of the synthetic resin configuring the outerwheel element 16 g and the minute concave parts 58 a and 58 bconfiguring the knurling surface 57.

In the case of this embodiment, the knurling surface 57 has a minuteconcave-convex shape (W_(58a)≤H_(20a)/10 and W_(58b)≤H_(20a)/10). Thus,the entire volume of the synthetic resin is hardly affected although aportion of the synthetic resin configuring the outer wheel element 16 genters into the minute concave parts 58 a and 58 b configuring theknurling surface 57. For this reason, even in the case of thisembodiment, in the outer wheel element 16 g, the portion which issuperimposed on the radially outer side with respect to the knurlingsurface 57 can be set to have almost (substantially) the same radialthickness in the portions in which the plurality of teeth 20 a and 20 aconfiguring the worm wheel tooth part 19 a are positioned. Accordingly,even in the case of this embodiment, the molding shrinkage amounts ofthe teeth 20 a and 20 a at the time of injection-molding the outer wheelelement 16 g can be set to be almost the same as each other. Thus, it ispossible to suppress the manufacturing error such as the pitch errorwith respect to the worm wheel tooth part 19 a.

The other configuration and effect are similar with the case of theabove-described first embodiment.

Eighth Embodiment

An eighth embodiment of the invention will be described with referenceto FIG. 17. This embodiment is a modification of the above-describedfirst embodiment illustrated in FIGS. 1 to 7.

In the case of the worm reduction gear of this embodiment, thecylindrical surface part 24 is provided only in an axial intermediateportion of an outer circumferential surface of an inner wheel element 15h configuring a worm wheel 4 h, and the entire engaging portion 50 ofthe worm tooth part 18 and the worm wheel tooth part 19 a is configuredto be radially superimposed with the cylindrical surface part 24.

In the case of this embodiment, the radially outer end parts (theportions which are positioned on the radially outer side from first andsecond annular concave parts 22 and 38) of the both axial surfaces ofthe inner wheel element 15 h are each provided with a concave-convexpart 23 b formed by alternately arranging a concave part 26 b and aconvex part 27 b in the circumferential direction. The concave-convexpart 23 b is provided in the axial range which is adjacent to both axialsides of the cylindrical surface part 24. Further, in the case of thisembodiment, when a portion of a synthetic resin configuring an outerwheel element 16 h enters into the concave parts 26 b configuring theconcave-convex parts 23 b and 23 b, the holding power of the outer wheelelement 16 h in the rotation direction with respect to the inner wheelelement 15 h is improved.

Even in the case of this embodiment having the above-describedconfiguration, the entire engaging portion 50 is radially superimposedwith the cylindrical surface part 24. Thus, the manufacturing error suchas the pitch error with respect to the worm wheel tooth part 19 a can besuppressed in the axial range (the axial range in which the cylindricalsurface part 24 is positioned) in which the engaging portion 50 ispositioned. Accordingly, the engaged state of the engaging portion 50can be made excellent.

Incidentally, in the illustrated example, the concave-convex part 23(see FIGS. 2 and 3) is not provided in the outer diameter sidecircumferential surface configuring the inner surface of the firstannular concave part 22. However, in a case where the invention isimplemented, the concave-convex part 23 can be provided (for example, inthe state of being continuous to the concave-convex part 23 b on theaxial one side).

The other configuration and effect are similar with the case of theabove-described first embodiment.

Ninth Embodiment

A ninth embodiment of the invention will be described with reference toFIGS. 18 and 19. This embodiment is a modification of theabove-described first embodiment illustrated in FIGS. 1 to 7.

In the case of this embodiment, in the outer diameter sidecircumferential surface configuring the inner surface of the firstannular concave part 22 provided in an axial one-side surface of aninner wheel element 15 i configuring a worm wheel 4 i, a sub concavepart 59 is provided to be recessed radially outward over the entirecircumference in an axial deep end part (which is an axial other endportion and a right end part in FIGS. 18 and 19) of the first annularconcave part 22. Further, the cross section of the sub concave part 59with respect to a virtual plane including the central axis of the innerwheel element 15 i has a V shape such that the width dimension in theaxial direction becomes smaller from an opening part on the innerdiameter side toward the bottom part on the outer diameter side.

In the case of this embodiment, the concave-convex part 23 is providedin the entire portion deviated from the sub concave part 59 in the outerdiameter side circumferential surface configuring the inner surface ofthe first annular concave part 22. Further, the axial deep end parts ofthe concave parts 26 configuring the concave-convex part 23 are openedto the inner surfaces of the sub concave parts 59, respectively.Incidentally, in the case of this embodiment, the radial depth of thesub concave part 59 is slightly larger than the radial depth of theconcave part 26 configuring the concave-convex part 23. Herein, in acase where the invention is implemented, the radial depth of the subconcave part 59 may be configured to be the same as the radial depth ofthe concave part 26 or be configured to be less than the radial depth ofthe concave part 26.

In the case of this embodiment, in a synthetic resin configuring anouter wheel element 16 i, a portion of the portion (the suppression part31 having an annular shape) having entered into the first annularconcave part 22 enters into the entire sub concave part 59 to cover theentire inner surface of the sub concave part 59, so as to configure asub suppression part 60 (which has a shape coinciding with the subconcave part 59) engaged with the sub concave part 59.

In the case of this embodiment having the above-described configuration,the holding power of the outer wheel element 16 i in the direction ofthe moment M with respect to the inner wheel element 15 i can beimproved on the basis of the engagement of the sub concave part 59 andthe sub suppression part 60.

In the case of this embodiment, as illustrated in FIG. 19, when theouter wheel element 16 i is injection-molded, the molten resin fed intothe cavity 45 through the runner 46 and the disc gate 47 reaches theportion corresponding to the suppression part 31 (sub suppression part60) and stops. For this reason, the sub suppression part 60 (sub concavepart 59) can prevent that the flow of the molten resin into the cavity45 is hindered. Particularly, in the case of this embodiment, the crosssection of the sub concave part 59 has a V shape such that the widthdimension in the axial direction becomes smaller from the opening parton the inner diameter side toward the bottom part on the outer diameterside. The molten resin can smoothly enter from the opening part of thesub concave part 59 toward the bottom part. Accordingly, it can beprevented that the flow of the molten resin into the cavity 45 ishindered, and the moldability of the sub suppression part 60 can be madeexcellent.

In the case of this embodiment, when the inner wheel element 15 i ismanufactured, after the sub concave part 59 is molded with respect tothe outer diameter side circumferential surface configuring the innersurface of the first annular concave part 22, in a case where theconcave-convex part 23 is molded by the plastic process such as coldforging, the sub concave part 59 may be used as an escape part of amolding die of the concave-convex part 23 or an escape part of a metalmaterial deformed according to the molding of the concave-convex part23. As a result, the molding load of the concave-convex part 23 issuppressed to be low, so as to reduce the molding facility capacity ofthe concave-convex part 23 and to improve the lifetime of the moldingdie. Herein, in a case where the invention is implemented, after theconcave-convex part 23 is molded, the sub concave part 59 may be moldedby the cutting process or the like.

The other configuration and effect are similar with the case of theabove-described first embodiment.

Incidentally, in a case where the invention is implemented, theinvention may be implemented by appropriately combining theconfigurations of the above-described embodiments.

In the above-described embodiment, the inner wheel element is formed ofmetal. However, in a case where the invention is implemented, forexample, the inner wheel element may be formed of a synthetic resinwhich is excellent in the thermal resistance compared to the syntheticresin configuring the outer wheel element. Also in this case, it ispossible to obtain the effect similar with the case of theabove-described embodiments.

In the structure of the above-described embodiment, in the surface ofthe inner wheel element, if at least one portion (for example, thecylindrical surface part and the entire surface of the inner wheelelement, and in the case of the above-described seventh embodiment, theportion deviated from the knurling surface 57) in the portions which arecovered with the synthetic resin configuring the outer wheel elementserves as a minute concave-convex surface formed by various kinds ofprocesses such as a knurling process, an emboss process, and a shotblast, a portion of the synthetic resin configuring the outer wheelelement enters into the concave part configuring the minuteconcave-convex surface. Thus, it is possible to improve the holdingpower (adhesiveness) of the outer wheel element with respect to theinner wheel element. Incidentally, also in a case where such aconfiguration is adopted, if the depth of the concave part configuringthe minute concave-convex surface is set to be equal to or less than onetenth (for example, equal to or less than one twentieth or equal to orless than one thirtieth) of the radial height of the teeth configuringthe worm wheel tooth part, so as to hardly make an effect on the volumeof the synthetic resin configuring the outer wheel element, it can besuppressed that the manufacturing error occurs in the portion engagedwith the worm tooth part in the worm wheel tooth part.

INDUSTRIAL APPLICABILITY

The worm wheel and the worm reduction gear of the invention are notlimited to the electric power steering device, but may be used invarious kinds of mechanical devices such as a wiper device in anassembled state.

The present application is based on Japanese Patent Application (No.2016-018233) filed on Feb. 2, 2016, Japanese Patent Application (No.2016-204200) filed on Oct. 18, 2016, and Japanese Patent Application(No. 2016-249614) filed on Dec. 22, 2016. The contents thereof areincorporated hereinto by reference.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

1 steering wheel

2 steering shaft

3 housing

4, 4 a to 4 i worm wheel

5 electric motor

6 worm shaft

7 output shaft

8 a, 8 b rolling bearing

9 torsion bar

10 torque sensor

11 a, 11 b universal joint

12 intermediate shaft

13 steering gear unit

14 pinion shaft

15, 15 a to 15 i inner wheel element

16, 16 a to 16 i outer wheel element

17 concave-convex part

18 worm tooth part

19, 19 a worm wheel tooth part

20, 20 a teeth

21 fitting hole

22, 22 a, 22 b first annular concave part

23, 23 a, 23 b concave-convex part

24, 24 a cylindrical surface part

25 a, 25 b flat surface part

26 26 a concave part

27 27 a convex part

28 inner diameter side annular part

29 outer diameter side annular part

30 connection part

31 first suppression part

32 rotation holding part

33 a, 33 b inner ring

34 a, 34 b outer ring

35 a, 35 b ball

36 cylindrical surface part

37 a, 37 b intermediate flat surface part

38, 38 a to 38 c second annular concave part

39 outer diameter side tilted surface part

40 outer diameter side cylindrical surface part

41 inner diameter side tilted surface part

42, 42 a, 42 b second suppression part

43 projecting part

44 molding device

45 cavity

46 runner

47 disc gate

48, 48 a inner diameter side cylindrical surface part

49 stepped surface

50 engaging portion

51 concave-convex part

52 concave part

53 convex part

54 concave-convex part

55 concave part

56 convex part

57 knurling surface

58 a, 58 b minute concave part

59 sub concave part

60 sub suppression part

1. A worm reduction gear comprising: a housing; a rotation shaft whichis supported to be rotatable with respect to the housing; a worm wheelwhich has a worm wheel tooth part in an outer circumferential surfaceand is externally fitted and fixed to the rotation shaft; and a wormshaft which has a worm tooth part in an axial intermediate portion of anouter circumferential surface and is supported to be rotatable withrespect to the housing in a state where the worm tooth part is engagedwith the worm wheel tooth part, wherein: the worm wheel includes: aninner wheel element; and an outer wheel element; the inner wheel elementhas a first annular concave part provided to be recessed in an axialdirection in an axial one-side surface; the outer wheel element is madeof a synthetic resin, and has a worm wheel tooth part in an outercircumferential surface, and the inner wheel element is embedded in theouter wheel element, such that a continuous range from an inner diameterside circumferential surface configuring an inner surface of the firstannular concave part, through an outer circumferential surface of theinner wheel element, to a portion near a radially inner end of an axialother-side surface of the inner wheel element in a surface of the innerwheel element is covered over an entire circumference; and the outercircumferential surface of the inner wheel element configuring the wormwheel is formed to be the cylindrical surface part in an axial rangewhich is radially superimposed with an entire of an engaging portion ofthe worm wheel tooth part and the worm tooth part.
 2. The worm reductiongear according to claim 1, wherein: a second annular concave part isprovided to be recessed in the axial direction in the axial other-sidesurface of the inner wheel element; and the inner wheel element isembedded in the outer wheel element, such that a continuous range fromthe inner diameter side circumferential surface configuring the innersurface of the first annular concave part through the outercircumferential surface of the inner wheel element to an inner diameterside circumferential surface configuring an inner surface of the secondannular concave part in the surface of the inner wheel element iscovered over an entire circumference.
 3. The worm reduction gearaccording to claim 2, wherein a tilted surface part which is tilted in adirection in which a width dimension in a radial direction of the secondannular concave part becomes larger toward an axial other side withrespect to a central axis of the inner wheel element is provided in atleast one circumferential surface among an outer diameter sidecircumferential surface and the inner diameter side circumferentialsurface configuring the inner surface of the second annular concavepart.
 4. The worm reduction gear according to claim 3, wherein thetilted surface part and a non-tilted surface part which is not tiltedwith respect to the central axis of the inner wheel element are providedin the outer diameter side circumferential surface configuring the innersurface of the second annular concave part.
 5. The worm eduction gearaccording to claim 3, wherein the tilted surface part and the non-tiltedsurface part which is not tilted with respect to the central axis of theinner wheel element are provided in the inner diameter sidecircumferential surface configuring the inner surface of the secondannular concave part.
 6. The worm reduction gear according to claim 2,wherein: in the inner wheel element, a portion which is positioned on aradially outer side from the first annular concave part and the secondannular concave part and a portion which is interposed between a bottomsurface of the first annular concave part and a bottom surface of thesecond annular concave part are each formed such that axial dimensionsof both portions which interpose a central position of the inner wheelelement in the axial direction are the same as each other; and in theouter wheel element, a portion which is positioned on a radially outerside from the inner wheel element and a portion which is superimposed inthe axial direction with respect to the portion which is positioned onthe radially outer side from the first annular concave part and thesecond annular concave part in the inner wheel element are each formedsuch that axial dimensions of both portions which interpose the centralposition of the inner wheel element in the axial direction are the sameas each other.
 7. The worm reduction gear according to claim 1, wherein:a concave-convex part in a circumferential direction is provided in thesurface of the inner wheel element; and a portion of a synthetic resinconfiguring the outer wheel element enters into a concave partconfiguring the concave-convex part.
 8. The worm reduction gearaccording to claim 7, wherein the concave-convex part is provided in theinner surface of the first annular concave part.
 9. The worm reductiongear according to claim 7, wherein: the second annular concave part isprovided to be recessed in the axial direction over the entirecircumference in the axial other-side surface of the inner wheelelement; and the concave-convex part is provided in the inner surface ofthe second annular concave part.
 10. The worm reduction gear accordingto claim 1, wherein: a sub concave part is provided to be recessedradially outward in a portion which is positioned on an axial deep sidefrom an axial opening-side end edge of the first annular concave part inthe outer diameter side circumferential surface configuring the innersurface of the first annular concave part; and a portion of thesynthetic resin configuring the outer wheel element enters into the subconcave part.
 11. The worm reduction gear according to claim 10, whereina cross section of the sub concave part with respect to a virtual planeincluding the central axis of the inner wheel element has a V shape suchthat a width dimension in the axial direction becomes smaller from anopening part on an inner diameter side toward a bottom part on an outerdiameter side.
 12. (canceled)
 13. The worm reduction gear according toclaim 11, wherein in the surface of the inner wheel element, at least aportion of portions which are covered with the synthetic resinconfiguring the outer wheel element serves as a minute concave-convexsurface.
 14. The worm reduction gear according to claim 1, wherein aprojecting part which projects on the axial other side from a portionadjacent to the radially outer side is provided in a radially inner endpart of an axial other-side surface of the outer wheel element. 15-17.(canceled)
 18. The worm reduction gear according to claim 1, wherein theentire outer circumferential surface of the inner wheel element servesas the cylindrical surface part.
 19. The worm reduction gear accordingto claim 1, wherein: a rolling bearing which includes an inner ring, anouter ring, and a plurality of rolling bodies provided between an outercircumferential surface of the inner ring and an inner circumferentialsurface of the outer ring, and supports the rotation shaft to berotatable with respect to the housing is provided in a portion adjacentto an axial other side of the worm wheel; an axial other-side surface ofthe outer wheel element configuring the worm wheel faces an axialone-side surface of the inner ring and an axial one-side surface of theouter ring in the axial direction; and an axial distance between theaxial other-side surface of the outer wheel element and the axialone-side surface of the inner ring is smaller than an axial distancebetween the axial other-side surface of the outer wheel element and theaxial one-side surface of the outer ring.
 20. The worm reduction gearaccording to claim 19, wherein: a projecting part which projects on theaxial other side from a portion adjacent to the radially outer side isprovided in a radially inner end part of the axial other-side surface ofthe outer wheel element; and an axial other-side surface of theprojecting part faces the axial one-side surface of the inner ring inthe axial direction.
 21. A manufacturing method of the worm reductiongear according to claim 1, the method comprising: positioning a radiallyouter end part of a disc gate in the radially inner end part of theaxial other side of the outer wheel element when an insertion molding isperformed in which the outer wheel element is coupled with the innerwheel element at the same time when the outer wheel element ismanufactured by an injection molding.